I've pulled a 5500 lb trailer with my delta 88 (3800 lb car, olds 307, 200R4 transmission, 2.73 rear), but I wouldn't reccomend it. If I were you, for stump-pulling torque in a small package, go with a 403 Olds. Gives you somewhere in the neighborhood of 275 lb-ft and 180 hp. These engines are great, the largest bore of any SBO as well as a pretty long stroke (I want to say somewhere in the neighborhood of 3.85"). Also, they can be built to withstand enormous amounts of power. Bill Trovato Racing just finished the Popular Hot Rodding Engine Master's Challenge in 10th place with 677HP and 567lb-ft torque on 92 octane. Granted, that's not the kind of animal you'd want in a street car, but it shows you that they can be built to make reliable power (he had about 75 dyno pulls going up to 6800 RPM on that motor by the time the EMC was over, and that was just during the competition!). 300 HP and 300 lb-ft torque is nothing with these motors.
If you want to keep the motor stock, throw one in with a quadrajet and 3.73's in the pumpkin and go towing. For transmissions, get a 200R4, but make sure it is built up enough to withstand the weight of the trailer. You'd have to do the same thing with a 700R4, so don't get conned into using one of those with an Olds motor. First, the bellhousing won't bolt to the motor without a conversion plate, and second, the 700R4's OD ratio is .70:1 where the 200R4's is .67:1.
BTW, 18-20 MPG with these should be no problem at all. Buddy of mine has an olds 455 in an 81 delta 88. With a 200R4, 3.73's and a quadrajet, a pretty mild 455 gets him 19 mpg on the highway.
